1. Field of the Invention
The present invention relates to a heat-sensitive material in which a diazonium salt compound and a coupler are used as color developing components and, especially, to a novel diazo-heat-sensitive material of a type developing a violet to cyan color which has excellent image preservation characteristics and image fixing characteristics.
2. Description of the Related Art
Along with improvement in the performance of heat-sensitive recording materials, there is great demand for the development of heat-sensitive materials which can develop a violet to cyan color, have a long shelf life of a recording material, and are improved in image preservation characteristics and image fixing characteristics.
Diazonium salt compounds, which have an outstandingly high chemical activity, react with a compound, which is called a "coupler", such as phenol derivatives, compounds containing an active methylene group, and the like, to produce azo dyes with ease. Also, these compounds possess photosensitivity and hence these compounds are decomposed by irradiation with light to lose the activity. For these reasons, the diazonium salt compounds have been utilized since early times as optical recording materials typified by diazo copying (see "Fundamentals of Photographic Engineering, Edition of Non-silver Salt Photography" edited by Japan Photographic Association, Published by Corona Co., Ltd., pp.89-117 and 182-201 (1982)).
Also, the diazonium compound has been lately applied to recording materials requiring image fixing by making use of its characteristics in which it is decomposed by light to lose its activity. Its typical examples which have been proposed include photo-fixing type heat-sensitive recording materials comprising a diazonium salt compound and a coupler which are heated according to an image signal to allow to react with each other and thereby to form an image, followed by irradiating the image with light to fix the image (see Kohji Sato, et al, J. Image Electronic Society, Vol. 11, No. 4, pp.290-296 (1982)).
These recording materials containing the diazonium salt compound as a color developing component have the drawback that the shelf life of the recording material is short because the activity of the diazonium salt is so high that the diazonium salt gradually decomposes, even in the dark, and loses its reactivity. As a measure for improving this drawback, a method in which the diazonium salt compound is encapsulated in a microcapsule is exemplified. With this method, the diazonium salt compound can be isolated from materials, such as water, a base, and the like, which promote decomposition, and hence the shelf life of the recording material can be prolonged greatly(see Toshimasa Usami, et al, J. Electrophotography Association, Vol. 26, No. 2, pp.115-125 (1987)).
In the case where such a microcapsule having a wall made of a material such as a urea resin or a urethane resin possessing a glass transition temperature above room temperature is used, the capsule wall exhibits non-material permeability at room temperature but exhibits material permeability above the glass transition temperature. Therefore, microcapsules of this type, called "thermally responsive microcapsules", are useful in heat-sensitive recording materials. Specifically, thermal responsive microcapsules containing the diazonium salt compound and a heat-sensitive recording layer containing a coupler and a base are applied to the surface of a substrate to form a recording material. With this structure, the diazonium salt compound can be kept for long periods in a stable manner and color images developed are formed readily by heating. Also, it is possible to fix the image by light irradiation.
As aforementioned, the microencapsulation ensures that the stability of the diazonium salt compound is outstandingly improved.
Meanwhile, it is known that 2-hydroxy-3-naphthoic acid anilides exhibit excellent performance as the heat-sensitive recording color forming material and create a blue color dye through a coupling reaction with a 4-substituted amino-2-alkoxybenzodiazonium salt compound (see Japanese Patent Application Laid-Open (JP-A) No. 2-225082).
However, the diazonium salt compound has the drawback that the use of a diazonium salt compound possessing .lambda..sub.max at a longer wavelength side causes a reduction in storability (coloring characteristics on background during storage prior to copying) . Also, there are the drawbacks that the use of a diazonium salt compound possessing .lambda..sub.max at a shorter wavelength side causes the fixing of an image to be hindered when irradiating with light if the above coupler is used and causes insufficiency in the preparation of a color of a longer wavelength extending to a cyan color and in the characteristics of preserving a developed color image (lightfastness).
As aforementioned, heat-sensitive recording materials, which possess the characteristics of developing a violet to cyan color and have excellent shelf life, image preservation characteristics, and image fixing characteristics, have not yet been developed.